Slurry recovery process

ABSTRACT

Frac sand from a well completion operation is transported to a recycling facility by truck. The frac sand is passed into a washing facility where is it separated from frac sand carrier residue, clays, drilling mud and metal particles. The wash water used in the washing facility is settled and re-used. Mud from the settled wash water is removed and re-used in drilling operations. The frac sand is removed from the washing facility by an inclined dehydration screw, and discharged into a counterflow hot air drying facility. Dust from the drying facility is collected and removed. Hot frac sand from the drying facility is passed through a chiller, and collected. The collected material is sized, and stored for re-use.

FIELD OF THE INVENTION

[0001] This invention relates generally to a process for the recovery and recycling of slurry materials produced in well drilling operations, such as frac sand or aqueous liquid for transporting frac sand, or both.

BACKGROUND OF THE INVENTION

[0002] A number of processes employ sand having specific qualities. In some cases the particular sand is only available from a small number of sources, and may have to be transported over relatively long distances, at significant expense, to the site where it is to be employed. The sand may have specific qualities with respect to size, hardness or strength, resistance to chemical attack, roundness, or other properties that are required for the particular process in question.

[0003] One such process involves the introduction of frac sand into wells under pressure. In the operation of a well, such as an oil or gas well, it may generally be desirable for the stratum, or strata, of the producing zone from which the oil or gas is to be extracted to have a number of cracks or fissures running generally away from the well bore. The general premise is that the larger the cracks, and the farther they run away from the well bore, the easier it is for the gas or oil in the stratum to migrate toward the well bore, whence it can flow, or be drawn, to the well-head at the surface.

[0004] One way to promote, or enhance, this process is to encourage the creation of a large number of cracks, and to try to force cracks in the stratum to open up, to lengthen, and to stay open so that the oil or gas can flow out. One method of doing this is to raise the pressure in the well, so that the adjacent cracks may tend to open up, and to force a “proppant” into the cracks the have been opened, so that when the raised pressure is removed, the cracks may tend to stay propped open. It is important that the propant be strong enough to keep the crack in the stratum open, and yet not fill the crack so tightly that the path for migration of oil or gas be thereby blocked. That is, the propant material must also let the oil and gas flow out.

[0005] One type of proppant material widely used in the oil and gas industry is a type of silica sand of a particular range of size and roundness. The roundness of the sand is advantageous since the grains, when stacked together, are interstitially porous. That is, the paths between the stacked, rounded grains, are sufficiently porous to permit the oil and gas to migrate through the spaces between the grains toward the well bore. It is not advantageous to use sharp particles that stack tightly together. The desirable, rounded sand of appropriate size is available from only a relatively small number of suppliers, and tends to be relatively expensive. When the propant insertion process is completed, the residue in the well bore is produced and, heretofore, discarded. Inasmuch as this residue contains a significant amount of frac sand propant, and inasmuch as frac sand is relatively expensive to buy and transport, it would be advantageous to recover frac sand from the residue produced from the well.

[0006] However, frac sand has not been recovered in the past for a number of reasons. First, the residue produced from the well includes not only frac sand, but may also include undesirable materials such as metal particles, drilling mud, and liquid frac sand carrier. The metal particles may tend to be steel filings worn from the drill string by the abrasive action of the frac sand, or other abrasive material, within the drill string itself. Alternatively the iron particles may be abraded from the casing of the bore, or from particles separated from the casing by a well perforating gun. The drilling mud tends to be a residue left over from the drilling of the well in the first place. The liquid frac sand carrier arises from the frac sand process itself.

[0007] The frac sand process involves mixing dry, clean frac sand with a carrier. It is important that the frac sand be distributed reasonably evenly in suspension in the carrier, as opposed to settling out, so that it will be relatively well dispersed for entering a multitude of cracks along the producing zone of the well bore. Therefore, at the time of mixing at the well-head, the carrier has the form of gel, or gel like substance in which the frac sand is suspended. The mixture is then forced down the well bore under high pressure. The pressure may tend to force the cracks in the producing stratum to open, and in opening to admit both the gel and the frac sand suspended in the gel. When the pressure is reduced to the pre-existing level, the cracks in the producing stratum attempt to close, but are held (that is, propped) open by the frac sand. Once the frac sand is in place, it is desirable that the gel carrier be removed. Therefore the gel has a limited duration, and after a time it returns to a thin, predominantly watery liquid form. As such it drains back into the well bore, and leaves most of the proppant sand behind. The excess proppant also remains in the well bore.

[0008] In due course, the material that drains into the well bore is pumped out, typically as a slurry of several components, namely: the liquid carrier; the excess frac sand; left-over drilling mud; and whatever else may have collected in the bore. In former times, the slurry produced from a new well at completion was extracted from the well and sprayed over terrain adjacent to the well head. This is not necessarily a desirable method of disposal, and has fallen out of favour in more recent times. The liquid carrier of the slurry from the completed well may well be a brine, and it is not desirable to spray various chlorides over adjacent agricultural lands. It would be preferable to recover the frac sand and to re-inject the iron laden aqueous solution back into the ground at an injection well.

[0009] While it is desirable to re-use the frac sand, it is undesirable to reintroduce into the well proppant sand that is contaminated with metal particles, since the metal particles may tend to then be forced into, and to block, the interstitial spaces in-between the sand granules that are needed to permit oil or gas to drain out. Similarly, it is undesirable to reintroduce proppant that is contaminated with drilling mud, since the drilling mud (and any undersized, cracked particles of proppant) will tend to block, i.e., stopper up, the cracks as well. It is thought that it may also be undesirable to re-introduce proppant that is contaminated with used carrier fluid. It would be desirable, then, to extract the frac sand residue, remove contaminants such as metal particles, drilling mud, broken or undersized particles of frac sand, or used carrier fluid, and dry the recovered frac sand so that it is ready for re-use. It would also be desirable to re-condition the aqueous solution that carries the frac sand to the surface to permit it to be discharged benignly as approved, or used in other operations, such as injection wells.

[0010] Injection wells have been used to extend production from existing wells. It may be that a producing well produces a mixture of water and oil. At the surface the water is separated from the oil. The oil is transported to a refinery, and the water remainder is then disposed. One form of disposal for the water is to re-inject it under pressure back into the oil or gas bearing stratum whence it came. The reinjected water is chemically treated before reinjection, the chemical treatment process typically including a biocide. Over a period of years, the repeated action of treating and re-injecting water may tend to result in a build up of sulphuric acidity in the water pumped from the producing well.

[0011] At the same time, the waste water from a new well frac sand completion process may tend to transport abraded iron particles that may tend to be small and to have a relatively high ratio of surface area to mass. Further, the solubility of iron in a salt solution may tend to be higher than in fresh water. If the aqueous solution of chlorides and iron from the new well, and low concentration sulphuric acid solution from the producing well are mixed, a black iron sulphate precipitate may tend to form. It is undesirable to direct this precipitate back into the injection well. First, the pumps of the injection well may be damaged by the precipitate, and second, over a period of time it is not desirable to clog the pores of the well with solid precipitate. In that light, it is desirable to treat the waste water by-product solution of the frac sand cleaning process to discourage the black precipitate from forming, and to mechanically filter the waste water output by passing it through a relatively fine filter. In particular, it may be desirable to introduce a modest concentration of citric acid, such as in a soluble anhydrous powder form, into the frac sand waste water output to form a soluble ferric citrate from which black ferric sulphate may have less tendency to form when the solutions are mixed.

[0012] Washing and drying systems for sand may tend to use washwater, and produce a quantity of dust. It would be advantageous to recycle at least part of the wash water, and thereby to reduce overall water consumption as compared to using only fresh water. It would also be advantageous to employ a dust collector, or collectors, to reduce the amount of dust given off in the process.

SUMMARY OF THE INVENTION

[0013] In an aspect of the invention there is a process comprising the steps of collecting frac sand after it has been used in a well bore, washing the frac sand, drying the frac sand and accumulating the washed and dried frac sand for re-use.

[0014] In an additional feature of that aspect of the invention, the process includes the step of segregating the frac sand by size before accumulating the washed and dried frac sand for re-use. In another additional feature, the process includes the step of mixing the washed and dried frac sand with a suspension agent and injecting the suspended frac sand into a well. In still another additional feature, the process further includes the step of collecting includes the step of collecting a slurry, a portion of the slurry being frac sand, settling the slurry, and decanting free liquid from the settled slurry.

[0015] In another aspect of the invention, there is a process for recovering used frac sand from a well, comprising the steps of extracting a slurry from the well, the slurry including at least frac sand and a carrier liquid, separating free liquid from the slurry to leave a first remainder, washing the first remainder in a washing facility, extracting a second remainder of wet solids from the washing facility, drying the wet solids, separating reusable, dried frac sand from other solids and accumulating the re-useable frac sand.

[0016] In an additional feature of that aspect of the invention, the step of washing includes introducing the first remainder into a tub. In another additional feature, the step of washing includes directing wash water at the first remainder. In still another additional feature, the process further includes the step of re-cycling at least a portion of the wash water. In yet another additional feature, the step of recycling includes the step of settling the used wash water. In still yet another additional feature, the step of settling the used wash water is followed by the step of decanting clear wash water for re-use.

[0017] In another additional feature, the step of setting includes introducing used wash water into a settling tank, allowing solids to settle out, and removing settled solids from the tank. In still another additional feature, the process further includes the step of chemically treating the wash water before directing it to the washing facility again. In a further additional feature, the process includes discarding a portion of the used wash water. In still yet a further additional feature, the process includes the step of directing an outflow of wash water from the washing facility along a path having at least one weir, and passing the flow over at least one weir.

[0018] In yet another additional feature, there is a settling zone upstream of the weir, and the process includes the step of passing the outflow through the settling zone on the way to the weir. In still another additional feature, there is a cascade of a plurality of weirs along the path, and the process includes removing precipitated material from upstream of the weirs. In yet another additional feature, the process includes the step of directing the outflow to a settling tank downstream of the weirs. In still another additional feature, the process includes decanting wash water for re-use from the settling tank. In another additional feature, the step of extracting includes raising the frac sand with an inclined screw.

[0019] In yet another additional feature, the step of drying includes the step of introducing at least a portion of the second remainder extracted from the washing facility into a rotating drum and introducing hot air into the drum to dry the second remainder. In a further additional feature, the step of drying includes passing exhaust air from the drum through a dust collector. In yet a further additional feature, the step of washing includes magnetic separation of metal particles from the slurry.

[0020] In yet another additional feature, the process includes the step of screening the solids to remove oversize particles. In still another additional feature, the process includes the step of screening the solids to exclude undersized particles. In a further additional feature, the process includes the step of screening the solids to exclude both oversize and undersize particles. In still yet a further additional feature, the process includes the step of screening the solids by size, and the step of screening includes the step of collecting dust arising from the step of screening.

[0021] In a further aspect of the invention, there is a process for recovering used frac sand from a well completion operation. The process includes the steps of extracting a slurry from the well, the slurry including at least frac sand, drilling mud, and a liquid carrier; passing at least a portion of the slurry through a washing facility to separate the drilling mud and the liquid carrier from at least a portion of the frac sand; removing at least the portion of the frac sand from the washing facility, drying at least the portion of the frac sand; and passing at least the portion of the frac sand through a sizing apparatus to yield a re-usable remainder.

[0022] In a still further aspect of the invention, there is a process for recovering re-usable frac sand from a hydrocarbon well completion operation. The process includes the steps of extracting a slurry from a well bore of a well, the slurry including a carrier liquid, frac sand, and drilling mud; passing at least a portion of the slurry into a washing facility; washing the drilling mud off the frac sand to yield a frac sand rich remainder; extracting the frac sand rich remainder from the washing facility; passing the frac sand rich remainder through a dryer; and passing the frac sand rich remainder through a sizing apparatus to yield re-usable washed, dried and sized frac sand.

[0023] In an additional feature of that aspect of the invention, the step of passing at least a portion of the slurry into the washing facility includes a step of passing the portion of the slurry through an input sizing apparatus to reject oversize solids, and then washing the portion of the slurry, less the oversize solids. In another additional feature, at least some of the slurry is at least partially settled to yield a free liquid portion and a solid rich portion, and the process includes the step of decanting the free liquid portion before introducing the solid rich portion of the slurry into the washing facility.

[0024] In a further additional feature, an input sizing apparatus is mounted to receive the portion of the slurry, the input sizing apparatus including a reciprocating screen. The washing facility includes a water delivery apparatus mounted to spray water over the portion of the slurry and the reciprocating screen. The process includes the steps of passing at least the portion of the slurry through the input sizing apparatus to reject off-spec solid material, spraying water on the solid rich portion as the solid rich portion is passed through the reciprocating screen.

[0025] In a yet further additional feature, the washing facility includes a magnetic element and the process includes the step of operating the magnetic element to extract ferro-magnetic particles.

[0026] In another feature, the washing facility includes a washwater supply, an outflow, and a settling system, the outflow being located to discharge into the settling system. The process includes the steps of providing washwater from the washwater supply to wash the drilling mud from the frac sand; and transporting at least a portion of the drilling mud in suspension in sand washwater through the outflow into the settling system. In still another feature, the process includes the steps of accumulating a sludge of drilling mud in the settling system and removing the accumulated sludge of drilling mud. In a further feature, the process may include re-using the drilling mud sludge in a down-hole drilling operation. In a still further feature, the outflow system includes a plurality of settling tanks arranged in series. A first of the settling tanks is located to receive the discharge from the outflow of the washing facility. A second of the settling tanks is located to receive a discharge from the first settling tank. A weir is located between the first and second settling tanks. The discharge from the first settling tank flows across the weir, and the process includes the step of skimming water from the settling system and re-using at least a portion of the water skimmed from the settling system.

[0027] In an additional feature, the process further includes the step of re-directing at least a portion of the water skimmed from the settling system to the washing facility. In another additional feature, the process further includes the step of directing at least a portion of the water skimmed from the settling system to an injection well.

[0028] In yet another feature, the washing facility includes a bath and a screw mounted at least partially within the bath, the screw being operable to urge the frac sand rich remainder from the bath toward the dryer. In a further additional feature, a chute is mounted to direct the frac sand rich remainder discharged from the screw to the dryer, and the process includes the stop of directing air from the dryer into the chute.

[0029] In still another feature, a dust extraction apparatus is connected to the dryer and the process includes the step of directing hot air exhaust from the dryer to through the dust extraction apparatus. In a further feature, a cooling apparatus is connected to receive at least a portion of the hot, dried frac sand rich remainder from the dryer and the process includes the step of passing the portion of the hot, dried frac sand rich remainder through the cooling apparatus. In still yet another feature, the second, or output, sizing apparatus includes a means for rejecting oversized particles, a means for rejecting undersized particles, and a discharge for “on-spec” particles and the process includes the step of accumulating and storing the “on-spec” particles for re-use.

[0030] In a still further aspect of the invention, there is a process for recovering re-usable frac sand from a hydrocarbon well completion operation. The process includes the steps of extracting a slurry from a well bore of a well, the slurry including a carrier liquid, frac sand, and drilling mud; settling the slurry to produce a free liquid portion and a solid rich portion; removing the free liquid portion; passing the solid rich portion through a first sizing apparatus to reject oversize solids; passing the solid rich portion, less the rejected oversize solids, into a washing facility; washing the drilling mud off the frac sand of the solid rich portion to yield a frac sand rich remainder; extracting the frac sand rich remainder from the washing facility; passing the frac sand rich remainder through a dryer; and passing the frac sand rich remainder through a second sizing apparatus to yield reusable washed, dried and sized frac sand.

[0031] In still another aspect of the invention, there is an apparatus for re-cycling used frac sand. The apparatus has machinery operable to collect used frac sand, and to transport the used frac sand; a washing facility for washing the frac sand, located to receive transported frac sand; a drying facility for drying the frac sand mounted to receive washed frac sand from the washing facility; a sizing machine for segregating the frac sand from off-spec material, the sizing machine being located to receive the frac sand from the drying facility; and a storage container for holding frac sand, the storage container being located to receive on-spec frac sand from the sizing machine.

[0032] In a still further aspect of the invention, there is the use for re-cycling of frac sand of an apparatus having machinery operable to collect used frac sand, and to transport the used frac sand; a washing facility for washing the frac sand, located to receive transported frac sand; a drying facility for drying the frac sand mounted to receive washed frac sand from the washing facility; a sizing machine for segregating the frac sand from off-spec material, the sizing machine being located to receive the frac sand from the drying facility; and a storage container for holding frac sand, the storage container being located to receive on-spec frac sand from the sizing machine.

[0033] In still another aspect of the invention there is a process for the treatment of slurry waste water from a well completion operation. The process includes the step of extracting a slurry from a well bore. The slurry includes at least an aqueous liquid, and solids suspended in the liquid; the solids including sand; iron in at least one form chosen from the set of: (a) iron in solution in the aqueous liquid, and (b) iron particles suspended in the aqueous liquid amongst other solids. The process further includes the steps of separating the aqueous liquid from at least the majority of the solids by using a mechanical separation apparatus to produce a remainder including at least some of the aqueous solution and some of the iron; converting at least some of the iron in the remainder to a compound form; mechanically filtering the remainder to remove iron particles from suspension to produce a treated output solution; mixing the treated output solution with a solution of re-cycled hydrocarbon field production water; and re-injecting the mixture of the treated output solution and the re-cycled hydrocarbon field production water into a hydrocarbon producing stratum.

[0034] In an additional feature of that aspect of the invention, the step of converting includes adding citric material to the remainder.

[0035] These and other aspects and features of the invention may be understood with the assistance of the Figures and description as provided hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 shows a plan view of a sand recovery installation according to the present invention;

[0037]FIG. 2A is a first portion of a pictorial schematic diagram of the sand recover installation of FIG. 1;

[0038]FIG. 2B is a second portion of a pictorial schematic diagram of the sand recover installation of FIG. 1;

[0039]FIG. 2C is a third portion of a pictorial schematic diagram of the sand recover installation of FIG. 1;

[0040]FIG. 2D is a fourth portion of a pictorial schematic diagram of the sand recover installation of FIG. 1; and

[0041]FIG. 2E is a fifth portion of a pictorial schematic diagram of the sand recover installation of FIG. 1; and

[0042]FIG. 3 shows a block diagram representation of a process employing the installation of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0043] The description which follows, and the embodiments described therein, are provided by way of illustration of an example, or examples of particular embodiments of the principles of the present invention. These examples are provided for the purposes of explanation, and not of limitation, of those principles and of the invention. In the description which follows, like parts are marked throughout the specification and the drawings with the same respective reference numerals. The drawings are not necessarily to scale and in some instances proportions may have been exaggerated in order more clearly to depict certain features of the invention.

[0044]FIG. 1 shows a plan view, or map, of a frac sand recovery installation, indicated generally as 20. It is located some distance from a well field 22 from which frac sand is to be recovered. A battery of wastewater injection wells 24 is also located some distance away in an old, existing producing well field, 23 (FIG. 2B). Old, exiting producing wells are indicated as 25. A road 26 links installation 20 with well field 22 and injection wells 24. A pipe system 28 also links injection wells 24 and frac sand recovery installation 20.

[0045] Operation of the frac sand recovery installation 20 is controlled and directed from a control trailer, 21. Describing installation 20 in the order in which material would be processed, input material arrives at the site of sand recovery installation 20 by truck 30. The material is generally obtained in one of two ways.

[0046] First, well 32 in producing field 22 has been subject to a well completion treatment to introduce frac sand. That is, well 32, typically being a new well, has been pressurised to open cracks in the adjacent mineral bearing stratum. Frac sand has been mixed with a gel carrier, and has been introduced under pressure into well 32, and forced under pressure into the cracks and fissures of the adjacent stratum. Then the pressure in the well has been reduced to its previous level. The carrier has reverted to a liquid form, and has drained back into the well, leaving the majority of the frac sand in place as a proppant to keep the cracks open. The remainder of the frac sand, the liquid carrier, and other matter, such as drilling mud with any metal shards or particles abraded from the drill string, collect in the well bore sump. The material from the well bore has been collected by producing it from the well using collection apparatus in the nature of pumps and collection tanks as may be suitable, and transferred to the frac sand recovery site using transport machinery such as truck 30. Truck 30 is most typically a vacuum truck for transporting the collected material in the form of a slurry.

[0047] Second, the collected material may have been recovered from an old pit or dump 34. That is, formerly, the slurry produced from wells after introduction of proppant has usually been produced from the well, and then discarded, typically being collected at the well head and then discharged into a pit or dump 34 from which the liquid eventually evaporates or drains away. As the opportunity to re-cycle potentially valuable frac sand is made available, the formerly discarded material can be mined from existing dump sites. Material collected, such as by mining from a dump site, can be transported in either vacuum truck 30 if wet, or in a gravel truck 36 if drained, as may be appropriate. While preferred, it is not necessary that the free water be drawn off. Decanting off the free water first may tend to reduce the quantity of liquid poured into the bins noted below.

[0048] If the material to be treated arrives in a vacuum truck 30, as is most typical, it will generally have settled during the trip, and vacuum truck 30 may be equipped with a vibration unit to enhance this settling. This may result in a first portion of the water, or watery liquid, in vacuum truck 30 forming a layer of “free water” from which the solids have settled out. As such, as a preliminary optional step, the free water is decanted off the slurry in vacuum truck 30 to a sloop tank 38. This leaves a first remainder in truck 30, the first remainder including the solids and the balance of the liquid remaining after the free water has been decanted off. The free water from well 32 may tend typically to be a salty chloride brine with iron in solution, or possible an amount of very fine iron particles in suspension.

[0049] Sloop tank 38 is a semi-buried open top tank having a liner—that is, tank 38 is equipped with an external secondary containment liner and has a monitoring well 40 placed at one end of the tank. Monitoring well 40 can monitor the interstitial space 42 between the tank and the liner. Water collected in sloop tank 38 can be removed for treatment elsewhere, can be directed into the wash-water system described below, or can be used elsewhere for other purposes, as may be permitted, or may be discarded in an approved manner.

[0050] Once the free water has been removed, the solids rich remainder of the material in the vacuum truck 30 (or the material in gravel truck 36, as may be) is weighed at weigh scale 44, adjacent to scale control trailer 45, and emptied at either of a pair of first and second offload pits 46, 48. Offload pits 46, 48 are also semi-buried tanks that each have a secondary containment liner and a leak detection system in the nature of an interstitial monitoring well, also indicated as 40, as noted above. Pits 46, 48 (i.e., the semi-buried tanks) are sloped to prevent any free fluids from running out. A monitoring well 40 is again positioned at the low point to permit effective leak detection. Truck 30, (or 36, as may be), now empty, is re-weighed, and the difference between the full and empty weights calculated, that difference being the weight of wet sand material deposited in the offload pit.

[0051] The material from offload pits 46, 48, is transported to an inlet hopper 52 of the washing facility, indicated generally as 50. Transport may be by a conveyor or, as in the preferred embodiment, by a front-end loader 54.

[0052] A conveying device in the nature of a rotating sand screw, or auger 56 feeds material from hopper 52 onto a reciprocating (that is, vibrating) wash deck 58 mounted above a tub 60 of washing facility 50 (FIG. 2A). In the preferred embodiment hopper 52 is a 10′×10′ hopper, and auger 56 is a 24 inch sand screw. A wash water delivery system 62 includes nozzles or water jets 64 located to direct an inflow of wash water against the solids rich material fed from auger 56 onto wash deck 58. The wet material, and the wash water, fall or drain, through a ½ inch grid screen 66 of wash deck 58, and into tub 60. Oversized material rejected by screen 66 such as stones inadvertently included in material recovered from old storage pits is discarded through a discharge chute to an oversize bin 68. Bin 68 is periodically emptied as required.

[0053] Tub 60 has two exits. The first exit is by an inclined dehydrating screw 70, described below. The second exit is by an outflow passage 72 leading to a settling system in the nature of a three compartment, high level overflow rig tank 74 (FIG. 2B). Tub 60 has a high level weir 71 leading to passage 72. Rig tank 74 has a series, or set, of weirs 76 and 78 of decreasing height. Although two such weirs are shown, a greater number could also be used. There are respective settling, or low velocity, zones 80, 82, 84, in each of the three compartments, in which a sludge of heavier solid particles 67 silts out over time. The outflow from tub 60 may tend generally to transport away the drilling mud in suspension, and the resultant accretion of sludge 67 collected upstream of weirs 76, 78 in settling zones 80, 82, 84 is largely, if not entirely, drilling mud. The collected mud is routinely removed from each of the three compartments.

[0054] The wash water is recycled through rig tank 74, on average, up to about four times. That is, the ratio of the flow through re-circulation pump 75 and re-circulation line 77 from the most downstream settling zone, namely zone 84, back to washing tub 60 is about four times as great (+/−) as the flow out through pump 85 and pipes 86. The wash water is skimmed off the rig tank and moved, (that is, pumped through pipes 86), into one of three alternately selectable waste water holding tanks, 88, 90, 92. Fines are allowed further time to settle in holding tanks 88, 90 and 92. While one tank is selected, waste water is being settled in one or both of the others. Tanks 88, 90 and 92 are periodically cleaned or flushed to remove accumulated fines.

[0055] The settled wastewater is treated for downhole compatibility with additives as shown schematically at 87, drawn from a settled tank, whichever it may be. The relatively clean, treated output wastewater is then passed through a filter 89 before being sent either to the battery of injection wells 24 for re-injection into the mineral bearing stratum, or for other use, or for discard in an approved manner. The chemical additives may include citric acid in a soluble anhydrous powder form such as may tend to combine with the iron in solution in the wastewater. The quantity of citric additive is also modest, in one embodiment being of the order of a quart per 400 bbl settling tank. The quantity of additive required may tend to vary according to the condition and dilution of the waste water as received from truck 30. Filter 89 may typically have filter elements for trapping solid particles of greater than 20 microns in size, the trapped material being removed at 91. The volume of water directed to the battery wells, or discarded, is measured and monitored for solid precipitants.

[0056] Treated, filtered wastewater sent by pipe 24 to old producing field 23 is mixed with treated, re-cycled water obtained when the material produced from old well 25 is separated into oil 95 and water 97. The recycled water from well 25 has been treated with biocide, and may tend to have a somewhat sour, sulphuric acidic nature, depending on the condition of well 25. The mixture is then injected into well 24 under pressure, as provided by pump 93.

[0057] Inasmuch as the metal particles tend to be predominantly, if not wholly, steel from the drill string, they also tend to be ferro-magnetic in nature. Metal particle removal apparatus, in the nature of magnetic heads 94 mounted at the outflow lip of high level weir 71 of tub 60 and at weirs 76 and 78, where the flow is shallow, to attract the metal particles that may have been entrained in the slurry. Magnetic heads 94 are periodically removed and cleaned of accumulated metallic material.

[0058] Returning to tub 60, the second exit is by a lifting device in the nature of inclined dewatering screw 70 mounted within an inclined channel, or casing 73 that is manufactured as an inclined extension of the main body of tub 60. The sand and wash water that fall through screen 66 fall on dewatering screw 70 and into tub 60. Over time, the use of the wash water and the agitation of the sand by dewatering screw 70 tends to clean the frac sand, such that the sand eventually raised by dewatering screw 70 and thus extracted from tub 60 is relatively clean frac sand.

[0059] The wet sand discharged from the upper end of dewatering screw 70 is directed through a chute 96 into a drying facility, namely rotating drum dryer 100 fed from chute 96 by auger 97 (FIG. 2C). In the preferred embodiment dryer 100 is 5 ft in diameter and 40 ft long. Dryer 100 is supplied with heated air by a burner 102. The inside of dryer 100 has baffles 104 that lift and turn the sand and allow it to fall in the heated airflow as the dryer drum 106 turns. The incline of the dryer is such as to urge the sand gently from the elevated inlet end at chute 96 to the somewhat lower outlet end at burner 102 over a period of time, such that the sand may tend to be dry by the time it reaches the exit.

[0060] In the preferred embodiment dryer 100 operates at about 700 F. Dryer 100 is a counter-flow dryer operating on a negative air system. That is, the direction of the airflow, from burner 102, through rotating drum 106, is generally in the opposite, or counter-flow direction to the progress of the sand that enters from chute 96. The sand proceeds along rotating drum 106 to a sand discharge auger 108 located adjacent to the air inlet opening 110 into which burner 102 is directed. The negative air system, rather than blowing air into dryer 100, draws it through dryer 100 at a negative pressure relative to ambient, by use of an air mover, or fan, in the nature of a centrifugal blower 112.

[0061] Blower 112 (FIG. 2D) is mounted to draw the heated air through drum 106, from inlet 110 at burner 102, as noted above, and to extract the hot air exhaust and entrained dust from the end of dryer 100 adjacent to the chute 96. The air and dust extracted are drawn first into a large, dry cyclone 114. Settled off-spec undersized sand particles and fines are discharged from dry cyclone 114 through a discharge chute 116 for collection in a receptacle, or containment bin in the nature of an above ground shale sloop 118, and either sent to a waste management facility or reused as may be appropriate.

[0062] The air exhaust from dry cyclone 114 is drawn through ducting 113 into a wet dust collection apparatus, or scrubber 122. In a first portion 115 of wet scrubber 122, the exhaust air from dry cyclone 114 is drawn downwardly through a large chamber 117 in which it has relatively low downward velocity. Water is drawn from a settling tank 124 through a pump 119 and piping 120 to be sprayed as a mist from an array of nozzles 121 into the passing still relatively warm, dry air. The now moistened air, and any unevaporated spray, then passes through a throat leading to a check valve 123. Check valve 123 permits the wet air to pass into the second portion of scrubber 122, namely a bath, 127 that is filled with water. When the pressure in bath 127 at check valve 123 is lower than in portion 115, the air, and entrained dust particles, will be drawn into bath 127, the air bubbling upward to the surface of the water. The water level of bath 127 is maintained by a spring loaded release valve 129 that operates to release water from bath 127 when the internal level exceeds a “full” height. The released water is returned to tank 124. Wet discharge from scrubber 122 passes through a solid discharge chute 131 to be collected in settling tank 124. The moisture laden air exhaust from wet scrubber 122 is extracted through ducting 126 by blower 112 and is discharged to the atmosphere through a stack 133.

[0063] A lifting device in the nature of a bucket elevator 128 (FIG. 2C) transports the hot, dry sand from the discharge of dryer 100 to the top of a cooling facility in the nature of a counter-flow cooling tower 130. Tower 130 has a sand inlet at the top, and a sand discharge at the bottom. An array of baffles 132 inside tower 130 causes the sand to fall through the airflow several times before reaching the bottom. Ambient, relatively cool air is introduced at the base of tower 130 and is drawn upward to encounter the cascading, downward falling sand. The air is forced out the top of tower 130 and then through a dust collector cyclone 134 by a blower 136. The fines from cyclone 134 are collected in an open ended tank 138, and are disposed of in a suitable manner.

[0064] The sand leaving the bottom end of cooling tower 130 is at a moderate temperature, in the range of 150 F., with variation depending on the ambient temperature. The sand leaving through the discharge chute of cooling tower 130 falls into a receptacle in the nature of an open ended tank 140, where it collects. The sand is then transported to a sizing facility in the nature of a system of moving screens, most preferably a rotary screener 142. Rotary screener 142 rejects remaining small stones and over spec sand. This leaves a remainder of washed, dried, on-spec sand that is transported by an elevator 144 to one of four storage receptacles in the nature of bins 146, 148, 150 and 152. This sand is ready to be loaded by a transport means such as movable conveyor 154 onto trucks, such as gravel truck 158, weighed on scale 44 (empty and full, as above, to permit the difference to be calculated) and sent back to well completion sites for use as frac sand. The oversize, off-spec sand from screener 142 is accumulated in a bin 156 and then transferred, typically by truck, for landfill or for use in sand and gravel operations more generally. Both the volume of material stored in bins 146, 148, 150 and 152 and the weight of material sent out from recovery facility 20 are monitored, as for example when departing trucks 158 with clean frac sand are weighed empty and full. The frac sand can then be used, as described above, by mixing with the gel carrier, introduction under pressure into a well, and so forth.

[0065] In this system, fresh water is provided from an external source, such as a lake or river, and is introduced by means of a pump 160 and pipe 162 at the wet scrubber setting tank 124, from which it is forced by pump 61 and line 63 pumped to wash water delivery system jets 64, as noted above. The decanted water from sloop tank 38 is pumped into the first settling region, 80, of rig tank 74 by means of a pump 164 and pipe 166.

[0066] Embodiments of the invention have now been described in detail. Since changes in and or additions to the above-described best mode may be made without departing from the nature, spirit or scope of the invention, the invention is not to be limited to those details, but only by the appended claims. 

I claim:
 1. A process comprising the steps of: extracting frac sand from a well bore; collecting the frac sand extracted from the well bore; washing the frac sand; drying the frac sand; sizing the frac sand; and accumulating the washed, dried and sized frac sand for re-use.
 2. The process of claim 1 wherein the process includes the step of segregating the frac sand by size before accumulating the washed and dried frac sand for re-use.
 3. The process of claim 1 further including the step of mixing the washed and dried frac sand with a suspension agent and injecting the suspended frac sand into a well.
 4. The process of claim 1 wherein the step of collecting includes the step of collecting a slurry, a portion of the slurry being frac sand, settling the slurry to yield a free liquid portion, and decanting the free liquid portion from the settled slurry.
 5. A process for recovering used frac sand from a well completion operation, said process including the steps of: collecting a deposit that includes frac sand and other materials; introducing at least the frac sand into a washing facility; washing said frac sand; discharging at least a portion of said frac sand from said washing facility to a drying facility; drying at least said portion of said frac sand; passing at least said portion of said frac sand through a sizing apparatus to yield a remainder of washed, dried, and sized frac sand for re-use.
 6. The process of claim 5 wherein the step of washing includes introducing the frac sand into a tub.
 7. The process of claim 6 wherein the step of washing includes directing wash water at the frac sand as it is falling into said tub.
 8. The process of claim 7 further including the step of re-cycling at least a portion of the wash water.
 9. The process of claim 8 wherein the step of recycling includes the step of settling used wash water.
 10. The process of claim 9 wherein the step of settling the used wash water is followed by the step of decanting clear, settled, wash water for re-use.
 11. The process of claim 9 wherein the step of setting includes introducing used wash water into a settling tank, allowing solids to settle out, and removing settled solids from the tank.
 12. The process of claim 8 further including the step of chemically treating the wash water for compatibility with recycled producing well water before directing it to an injection well.
 13. The process of claim 8 including discarding a portion of the used wash water.
 14. The process of claim 8 including the step of directing an outflow of wash water from the washing facility along a path having at least one weir, and passing said flow over said at least one weir.
 15. The process of claim 14 wherein there is a settling zone upstream of said weir, and said process includes the step of passing the outflow through said settling zone on the way to said weir.
 16. The process of claim 14 wherein there is a cascade of a plurality of weirs along said path, and said process includes removing precipitated material from upstream of said weirs.
 17. The process of claim 16 wherein said process includes the step of directing the outflow to a settling tank downstream of said weirs.
 18. The process of claim 16 wherein said process includes decanting wash water for re-use from said settling tank.
 19. The process of claim 5 including the step of raising said frac sand from the tub with an inclined screw.
 20. The process of claim 5 wherein the step of drying includes the step of introducing at least a portion of said frac sand discharged from said washing facility into a rotating drum and introducing hot air into the drum to dry the frac sand.
 21. The process of claim 20 wherein the step of drying includes passing exhaust air from the drum through a dust collector.
 22. The process of claim 5 wherein said step of washing includes magnetic separation of metal particles from said slurry.
 23. The process of claim 5 wherein the process includes the step of screening the solids to remove oversize particles.
 24. The process of claim 5 wherein the process includes the step of screening the solids to exclude undersized particles.
 26. The process of claim 5 wherein the process includes the step of screening the solids to exclude both oversize and undersize particles.
 27. The process of claim 5 wherein said process includes the step of screening the solids by size, and the step of screening includes the step of collecting dust arising from the step of screening.
 28. A process for recovering used frac sand from a well completion operation, said process comprising the steps of: extracting a slurry from the well, the slurry including at least frac sand, drilling mud, and a liquid carrier; passing at least a portion of the slurry through a washing facility to separate said drilling mud and said liquid carrier from at least a portion of said frac sand; removing at least said portion of said frac sand from said washing facility, drying at least said portion of said frac sand; passing at least said portion of said frac sand through a sizing apparatus to yield a re-usable remainder.
 29. A process for recovering re-usable frac sand from a hydrocarbon well completion operation, said process comprising the steps of: extracting a slurry from a well bore of a well, the slurry including a carrier liquid, frac sand, and drilling mud; passing at least a portion of the slurry into a washing facility; washing said drilling mud off said frac sand to yield a frac sand rich remainder; extracting said frac sand rich remainder from said washing facility; passing said frac sand rich remainder through a dryer; and passing said frac sand rich remainder through a sizing apparatus to yield re-usable washed, dried and sized frac sand.
 30. The process of claim 29 wherein said step of passing at least a portion of said slurry into said washing facility includes a step of passing said portion of said slurry through an input sizing apparatus to reject oversize solids, and then washing said portion of said slurry, less the oversize solids.
 31. The process of claim 29 wherein at least some of said slurry is at least partially settled to yield a free liquid portion and a solid rich portion, and said process includes the step of decanting the free liquid portion before introducing the solid rich portion of the slurry into the washing facility.
 32. The process of claim 29 wherein: an input sizing apparatus is mounted to receive said portion of said slurry, said input sizing apparatus including a reciprocating screen; said washing facility includes a water delivery apparatus mounted to spray water over said portion of said slurry and said reciprocating screen; and said process includes the steps of passing at least said portion of said slurry through said input sizing apparatus to reject off-spec solid material, spraying water on said solid rich portion as said solid rich portion is passed through said reciprocating screen.
 33. The process of claim 29 wherein said washing facility includes a magnetic element and said process includes the step of operating said magnetic element to extract ferro-magnetic particles.
 34. The process of claim 29 wherein said washing facility includes a washwater supply, an outflow, and a settling system, said outflow being located to discharge into said settling system, and said process includes the steps of providing washwater from said washwater supply to wash said drilling mud from said frac sand; and transporting at least a portion of said drilling mud in suspension in sand washwater through said outflow into said settling system.
 35. The process of claim 34 further comprising the steps of accumulating a sludge of drilling mud in said settling system and removing the accumulated sludge of drilling mud.
 36. The process of claim 35 further comprising re-using the drilling mud sludge in a down-hole drilling operation.
 37. The process of claim 34 wherein said outflow systems includes a plurality of settling tanks arranged in series, a first of said settling tanks being located to receive the discharge from said outflow of said washing facility, a second of said settling tanks being located to receive a discharge from said first settling tank, and a weir located between said first and second settling tanks, said discharge from said first settling tank flowing across said weir, and said process including the step of skimming water from said settling system and re-using at least a portion of the water skimmed from said settling system.
 38. The process of claim 37 wherein said process further comprises the step of redirecting at least a portion of the water skimmed from said settling system to said washing facility.
 39. The process of claim 37 wherein said process further comprises the step of directing at least a portion of the water skimmed from said settling system to an injection well.
 40. The process of claim 29 wherein: the washing facility includes a bath and a screw mounted at least partially within said bath, said screw being operable to urge said frac sand rich remainder from said bath toward said dryer.
 41. The process of claim 40 wherein a chute is mounted to direct said frac sand rich remainder discharged from said screw to said dryer, and said process includes the stop of directing air from said dryer into said chute.
 42. The process of claim 29 wherein a dust extraction apparatus is connected to said dryer and said process includes the step of directing hot air exhaust from said dryer to through said dust extraction apparatus.
 43. The process of claim 29 wherein a cooling apparatus is connected to receive at least a portion of the hot, dried frac sand rich remainder from said dryer and said process includes the step of passing said portion of the hot, dried frac sand rich remainder through said cooling apparatus.
 44. The process of claim 29 wherein said sizing apparatus includes a means for rejecting oversized particles, a means for rejecting undersized particles, and a discharge for “on-spec” particles and said process includes the step of accumulating and storing said “on-spec” particles for re-use.
 45. A process for recovering re-usable frac sand from a hydrocarbon well completion operation, said process comprising the steps of: extracting a slurry from a well bore of a well, the slurry including a carrier liquid, frac sand, and drilling mud; settling said slurry to produce a free liquid portion and a solid rich portion; removing the free liquid portion; passing said solid rich portion through a first sizing apparatus to reject oversize solids; passing said solid rich portion, less said rejected oversize solids, into a washing facility; washing said drilling mud off said frac sand of said solid rich portion to yield a frac sand rich remainder; extracting said frac sand rich remainder from said washing facility; passing said frac sand rich remainder through a dryer; and passing said frac sand rich remainder through a second sizing apparatus to yield re-usable washed, dried and sized frac sand.
 46. An apparatus for re-cycling used frac sand, the apparatus comprising: machinery operable to collect used frac sand, and to transport the used frac sand; a washing facility for washing the frac sand, located to receive transported frac sand; a drying facility for drying the frac sand mounted to receive washed frac sand from the washing facility; a sizing machine for segregating the frac sand from off-spec material, the sizing machine being located to receive the frac sand from the drying facility; and a storage container for holding frac sand, the storage container being located to receive on-spec frac sand from the sizing machine.
 47. The use for re-cycling of frac sand of an apparatus comprising: machinery operable to collect used frac sand, and to transport the used frac sand; a washing facility for washing the frac sand, located to receive transported frac sand; a drying facility for drying the frac sand mounted to receive washed frac sand from the washing facility; a sizing machine for segregating the frac sand from off-spec material, the sizing machine being located to receive the frac sand from the drying facility; and a storage container for holding frac sand, the storage container being located to receive on-spec frac sand from the sizing machine.
 48. A process for the treatment of slurry waste water from a well completion operation, the process comprising the steps of: extracting a slurry from a well bore, the slurry including at least an aqueous liquid, and solids suspended in the liquid; the solids including sand; the slurry including iron in at least one form chosen from the set of: iron in solution in the aqueous liquid; and iron particles suspended in the aqueous liquid amongst other solids; separating the aqueous liquid from at least the majority of the solids by using a mechanical separation apparatus to produce a remainder including at least some of the aqueous solution and some of the iron; converting at least some of the iron in the remainder to a compound form; mechanically filtering the remainder to remove iron particles from suspension; to produce a treated output solution; mixing the treated output solution with a solution of re-cycled hydrocarbon field production water; and re-injecting the mixture of the treated output solution and the re-cycled oil field production water into a hydrocarbon producing stratum.
 49. The process of claim 48 wherein said step of converting includes adding citric material to the remainder.
 50. The process of claim 48 wherein said process includes the step of encouraging separation of iron particles from said aqueous liquid magnetically. 